gh-138223: Fix Infinite loop in email._header_value_parser._fold_mime_parameters when parameter names are too long

Lib/email/_header_value_parser.py

@@ -3081,13 +3081,17 @@ def _fold_mime_parameters(part, lines, maxlen, encoding):
                 # the RFC standard width.
                 maxlen = 78
             splitpoint = maxchars = maxlen - chrome_len - 2
-            while True:
+            splitpoint = max(1, splitpoint)  # Ensure splitpoint is always at least 1
+            while splitpoint > 1:
                 partial = value[:splitpoint]
                 encoded_value = urllib.parse.quote(
                     partial, safe='', errors=error_handler)
                 if len(encoded_value) <= maxchars:
                     break
                 splitpoint -= 1
+            # If we still can't fit, force a minimal split
+            if splitpoint <= 1:
+                splitpoint = 1
             lines.append(" {}*{}*={}{}".format(
                 name, section, extra_chrome, encoded_value))
             extra_chrome = ''

Lib/email/header.py

@@ -527,6 +527,8 @@ def _append_chunk(self, fws, string):
                     continue
                 break
             else:
+                # No suitable split point found. Handle the case where we have
+                # an extremely long string that can't be split.
                 fws, part = self._current_line.pop()
                 if self._current_line._initial_size > 0:
                     # There will be a header, so leave it on a line by itself.

Misc/NEWS.d/next/Library/2025-01-27-12-00-00.gh-issue-138223.email-infinite-loop.rst

@@ -0,0 +1 @@
+Fix infinite loop in email.message.EmailMessage.as_string() when add_attachment() is called with very long parameter keys.